This invention is both a catalyst and a process for producing elemental bromine from hydrogen bromide using that catalyst.
Bromine is a chemical feedstock often used for the production of bromoalkanes or olefins from alkanes. Bromine is found in nature only as dilute sources such as seawater or as brine well deposits. The classic process for producing bromine from such sources involves a multistage process involving electrolyzing, chlorinating, or acidifying the brine to release elemental bromine or hydrogen bromide into a solution, aerating or steaming the resulting dilute solution, absorbing the bromine or hydrogen bromide from the aeration effluent, and distilling the resulting absorbate to recover the bromine.
It is, of course, desirable not to be placed in the position of requiring fresh bromine if another suitable bromine-containing source is available from which to produce elemental bromine. Hydrobromic acid or hydrogen bromide (HBr) is a byproduct of a wide variety of chemical processes. This invention is a process utilizing a hydrogen bromide feed in producing elemental bromine.
There are a number of processes described in the open literature which produce bromine according to the equation: EQU 4HBr+O.sub.2 .fwdarw.2Br.sub.2 +2H.sub.2 O.
One such process (British Patent 930,341) involves the conversion of hydrobromic acid solutions using dissolved metal ion catalysts. The soluble metal may be gold, cerium, chromium, nickel, platinum, thorium, titanium, or vanadium; but preferably is iron or copper. A gas containing oxygen is passed through the acidic solution containing HBr and the dissolved metal, all at a temperature below the boiling point of the liquid. The gaseous effluent is then separated via condensation and distillation into product bromine, water, and HBr for recycle to the oxidation step.
Similarly, U.S. Pat. No. 3,179,498, to Harding et al, discloses a process in which a nitrite catalyst is employed in an acidic, aqueous solution of HBr to effect the oxidation of the HBr to Br.sub.2. The temperature of the liquid is maintained between 0.degree. and 100.degree. C. Although any inorganic or organic nitrite is said to be suitable, preferred catalysts are alkali metal or alkaline earth metal nitrites.
There are a number of processes which use heterogeneous catalysts to effect the conversion of HBr to Br.sub.2.
U.S. Pat. No. 2,536,457, to Mugdan, teaches such a process. The conversion is carried out at a temperature between 800.degree. and 1200.degree. C. (preferably between 800.degree. and 1000.degree. C.) with an excess of oxygen. The catalyst is preferably cerium oxide and may be supported on pumice granules or other refractory materials. If excessive water is included in the reactor, a combustible gas such as hydrogen is included to maintain the reaction temperature. Clearly the reaction temperature for this process is quite high.
U.S. Pat. No. 3,273,964, to De Rosset, shows a process in which the effluent from a dehydrobromination reaction is contacted with a catalyst-adsorbent composite. The effluent contains olefinic hydrocarbons and is produced by a series of steps in which an alkane is brominated to form a bromoalkane; the bromoalkane is then dehydrobrominated to form the effluent of olefinic hydrocarbons and HBr. The catalyst-adsorbent composite adsorbs the HBr in a first step and, during regeneration, catalyzes the oxidation of HBr to form the desired Br.sub.2. The composite contains an adsorbent of a basic metal oxide such as magnesium, calcium, or zinc oxide, and a catalyst of a Group IV-B metal oxide such as titania, magnesia, or zirconia. The preferred composite contains magnesia and zirconia in a ratio from about 0.5:1 to about 5:1.
U.S. Pat. No. 3,260,568, to Bloch et al, teaches a process it, which a stream containing substantially dry HBr with a solid adsorbent containing a metal "subchloride" and which is the reaction product of a refractory metal oxide and a metal chloride. The contact takes place at conditions where the HBr replaces at least a portion of the chloride in the adsorbent. When the adsorbent has reached about six percent by weight, the adsorbent is regenerated by contacting it with a dry hydrogen chloride gas. The patent does not appear to suggest the conversion of the adsorbed HBr to Br.sub.2. The adsorbent is suggested to be selected from metal chlorides such as aluminum, antimony, beryllium, iron, gallium, tin, titanium, and zinc chlorides.
U.S. Pat. No. 3,310,380, to Lester, discloses a process for the adsorption of combined bromine (e.g., HBr and alkyl bromides) on a catalytic-adsorbent composite, recovering unsaturated hydrocarbons, and when the adsorbent is filled, contacting the composite with an oxygen-containing gas at a temperature between 50.degree. and 450.degree. C. to produce a Br.sub.2 stream also containing water and unreacted HBr. This stream (also in admixture with an oxygen-containing gas) is then contacted with a second stage reactor also containing the composite but at a temperature between 200.degree. and 600.degree. C. The composite in the first stage comprises, preferably, 0.5 to 10% by weight of copper or cerium oxide composited on magnesium oxide: the second stage composite comprises, preferably, 2.0 to about 50% by weight of copper or cerium oxide composited on an alumina or zirconia support.
Similarly, U.S. Pat. No. 3,346,340, to Louvar et al, suggests a process for the oxidation of HBr to Br.sub.2 using a catalyst-inert support composite. The composite comprises a copper or cerium oxide on an inert support having a surface area between 5 and 100 square meters per gram and containing less than about 50 micromoles of hydroxyl per gram. The supports may be alpha- or theta-alumina or zirconia. The preferred temperature is between 300.degree. and 600.degree. C.
U.S. Pat. No. 3,353,916, to Lester, discloses a two stage process for oxidizing HBr to form Br.sub.2 by the steps of mixing the HBr-containing gas with an oxygen-containing gas and passing the mixture at a temperature of at least 225.degree. C. over a catalyst selected from the oxides and salts of cerium, manganese, chromium, iron, nickel, and cobalt and converting a major portion of the HBr to Br.sub.2. The partially converted gas, still containing excess oxygen, is then passed through a second stage catalyst comprising a copper oxide or salt at a temperature of at least about 225.degree. C. but not exceeding a "catalyst peak temperature" of 350.degree. C. to convert the remaining HBr. The preferred support appears to be zirconia.
This two-stage arrangement is carried out to prevent loss of the copper catalyst. Because the preferred copper oxide is apparently converted to copper bromide during the course of the reaction and copper bromide volatilizes at "temperatures in excess of about 350.degree. C.", the "copper bromide migrates through the catalyst mass in the direction of flow with eventual loss of copper bromide and premature deactivation." Use of a first catalyst stage which is tolerant of high temperatures, although apparently not as active a catalyst as is copper, allows a cooler second catalyst stage containing copper to complete "quantitative conversion of bromine from hydrogen bromide."
U.S. Pat. No. 3,379,506, to Massonne et al, teaches a process for the selective oxidation of hydrogen bromide to bromine in the presence of fluorocarbons by passing the mixture of gases over a Deacon catalyst at a temperature of 250.degree. to 500.degree. C., preferably between 300.degree. and 400.degree. C. The Deacon catalyst is said to be a "mostly porous carrier such as pumice, alumina, silica gel, clay, or bentonite, impregnated with a solution of bromides or chlorides of metals such as copper, iron, titanium, vanadium, chromium, manganese, cobalt, molybdenum, tungsten, or mixtures thereof." The preferred catalyst is said to be a chloride of copper. The patent notes that:
"[a] very efficient and stable catalyst is an oxidation catalyst which is prepared by impregnating active alumina with chlorides of copper, rare earths and/or alkali metals, drying at about 120.degree. C. and subsequent activation at a temperature of 300.degree. to 450.degree. C."
One example shows the production and use of a catalyst of alumina, potassium, copper, and an amount of "rare earths of the cerite group as chlorides".
Another patent which notes the problem with the volatilization of copper bromide in the oxidation of hydrogen bromide to bromine is U.S. Pat. No. 3,437,445, to Hay et al. The solution is to eliminate the copper in favor of a noble metal, such as platinum and palladium. The reaction is carried out at a temperature between about 175.degree. and about 700.degree. C. with a contact time of at least about 0.1 sec, "but for best operation a contact time of about five and 25 seconds is preferred." The yield of bromine is only between 28 and 78 molar percentage.
U.S. Pat. No. 4,131,626, to Sharma et al, suggests a process in which bromide salts are heated in the presence of an oxygen-containing gas, silicon dioxide, and an oxidation catalyst at a temperature of about 500.degree. to 1000.degree. C. The bromine is produced in conjunction with sodium silicate.
None of these documents suggest a catalytic HBr oxidation process in which the catalyst comprises cerium bromide on a zirconia support. Furthermore, none of those disclosures show a process in which the cerium bromide is as stable nor produces Br.sub.2 in as efficient a yield as is done by our process.